An operating system (OS), for example, as disclosed in PTL1, provides a virtual memory space larger than the capacity of a main memory of a computer for each process using a virtual memory function. Generally, a virtual memory space is divided into areas having a fixed length (for example, 4 KB) called pages, and the main memory of a computer is allocated in units of the pages. Hereinafter, a divided area of the virtual memory space will be referred to as a “virtual page”. Accordingly, data (hereinafter, referred to as “memory data”) that is written into the virtual memory by a user program corresponding to a process is stored in an actual area (hereinafter, referred to as a “real page”) of the main memory allocated to the page. Then, among this memory data, by saving memory data having a low possibility of being accessed (including at least being read or written) by a user program in an HDD, a virtual memory space larger than the capacity of the main memory of the computer described above is provided for a process. A general user program tends to have a relatively small amount of memory data accessed in a short time, in other words, to have access locality. Accordingly, there are many OS's using least recently used (LRU) as an algorithm for determining a virtual page having a “low possibility of being accessed” described above.